This invention relates to a voice privacy system. More particularly it relates to a secure system for the electronic transmission of speech and other analog signals. The system incorporates a frequency shift arrangement in which the input signals are shifted up and down in frequency in series of small steps in accordance with a pseudo-random sequence. It thus accomplishes large overall frequency shifts without the unduly spurious signals that result from single-step shifts of the same magnitude.
The invention is directed primarily to the prevention of eavesdropping on voice transmissions. To prevent the unauthorized reception of voice transmissions the signals are often scrambled at the transmitting end and unscrambled or reconstituted at the receiving end, which is provided with the appropriate "key" for this decoding process. Unauthorized listeners, on the other hand, do not have the key and, therefore, ideally they are unable to unscramble the transmission.
In practice of course, all scrambled transmissions can be unscrambled by unauthorized recipients, given sufficient time and equipment sophistication, the amount of time and the degree of equipment complexity depending on the complexity of the scrambling. For voice transmissions of the type with which we are primarily concerned, security is required for at most a few hours after transmission and the information being communicated is generally not so valuable as to cause an eavesdropper to spend large sums of money in code-breaking equipment. Therefore, simple scrambling techniques can be used, resulting in a relatively low cost for the scrambling and unscrambling circuitry.
One of these simple scrambling techniques is the use of frequency inversion combined with variable frequency shifting. The voice signal is heterodyned with the output of a beat frequency generator whose frequency is just above the voice band. The upper sideband of the resulting signal is filtered out, leaving the lower sideband, which is in the voice band and has the original voice signal, but with an inversion of the frequencies thereof.
To lend further complexity to the scrambling, the beat frequency is shifted up and down according to a prearranged program and the same program is used at the receiving end to reconstitute the voice signals. An unauthorized recipient must be able to follow these shifts in the beat frequency in order to unscramble the signal. For the system to work effectively, the shifts in frequency must be fairly substantial, for example 1500 hz with a nominal beat frequency of 3000 hz. Moreover they must occur rapidly enough so that they cannot be followed manually by means of a tuning knob operated by an eavesdropper. In prior systems the resulting abrupt, large-magnitude changes in the beat frequency have resulted in the generation of spurious signals which unduly degrade the information-bearing (voice) signals. The present invention is directed to the correction of this deficiency of prior beat-frequency-shifting systems.